Ultra-high-strength steel sheet having excellent yield ratio and workability

ABSTRACT

An ultra-high-strength steel sheet having a component composition that includes specific amounts of each of C, Si, Mn, and Al and a remainder of iron and unavoidable impurities, and in which the amounts of each of P, S, and N among the unavoidable impurities are limited to a specific amount. The ultra-high-strength steel sheet includes 1 area % or more of a region in which martensite constitutes 90 area % or more, residual austentite constitutes 0.5 area % or more, and the local Mn concentration is at least 1.2 times that of the Mn content of the entire steel sheet. The ultra-high-strength steel sheet has a tensile strength of 1470 MPa or more, a yield ratio of 0.75 or more, and a total elongation of 10% or more.

TECHNICAL FIELD

The present invention relates to an ultra-high-strength steel sheetexcellent in yield ratio and workability. The steel sheet type of theultra-high-strength steel sheet in accordance with the present inventionshall be considered to include not only cold-rolled steel sheets, butalso various plated steel sheets such as hot-dip galvanized steel sheetsand hot-dip galvanized and alloyed steel sheets.

BACKGROUND ART

For the purpose of improvement of fuel consumption by weight reductionof vehicle bodies, steel sheets used for skeleton components ofautomobiles have recently been required to be increased in strength, andin order to ensure collision safety, a high yield ratio is alsorequired. On the other hand, in order to form parts with complicatedshapes, excellent workability is also required.

It has therefore been eagerly desired to provide an ultra-high-strengthsteel sheet increased in elongation (EL) while having a high yieldratio. More specifically, a steel sheet having a tensile strength of1470 MPa or more, a yield ratio of 0.75 or more, and an elongation of10% or more has been required.

In addition, although steel sheets for automobiles are subjected towelding during assembly of vehicle bodies or during mounting of parts,weldability heavily depends on compositions of the steel sheets. Inparticular, when C and Mn are added in large amounts, it is known thatthe weldability is degraded. It has therefore been required for thesteel sheets for automobiles to fulfill the above-mentioned mechanicalproperties, while having a composition satisfying 0.35 mass % or less ofC and 1.5 mass % or less of Mn.

Conventionally herein, in order to increase the elongation of thehigh-strength steel sheet, mainly the following two means have beenused.

(1) The amount of residual austenite is increased to utilize a TRIPaction thereof.

(2) The amount of soft ferrite (including bainitic ferrite) isincreased.

However, in order to allow a large amount of austenite to remain, themeans of the above (1) requires the increase of the added amount of C orMn, resulting in a failure to satisfy C≤0.35 mass % and Mn≤1.5 mass %.There has been therefore a problem that sufficient weldability cannot beensured.

On the other hand, in order to ensure the elongation, the means of theabove (2) requires a predetermined amount of a soft phase, resulting ina failure to satisfy a yield ratio of 0.75 or more. There has beentherefore a problem that sufficient collision safety cannot be ensured.

For example, Patent Literature 1 proposes a steel sheet that isincreased in resistance to hydrogen embrittlement and is also excellentin resistance to delayed fracture at a punching hole processing part, inan ultra-high-strength region having a tensile strength of 1180 MPa ormore, by allowing a large amount of austenite to remain by increasingthe Mn content in the steel sheet.

However, with respect to the above-mentioned steel sheet, the Mn contentin the steel sheet is more than 1.5 mass % for all the invention steelsas shown in the examples thereof, and there has been room forimprovement in terms of the weldability.

In addition, Patent Literature 2 proposes a steel sheet that can realizea tensile strength of 1470 MPa or more and an elongation of 10% or more,in a composition satisfying 0.35 mass % or less of C and 1.5 mass % orless of Mn, by increasing the fraction of a soft ferrite phase.

However, the above-mentioned steel sheet cannot realize a yield ratio of0.75 or more as shown in the examples thereof, and there is a problemthat sufficient collision safety cannot be ensured.

CITATION LIST Patent Literatures

Patent Literature 1: JP-A-2008-81788

Patent Literature 2: JP-A-2010-90432

SUMMARY OF INVENTION Technical Problems

Therefore, an object of the present invention is to provide anultra-high-strength steel sheet excellent in yield ratio andworkability, which can satisfy a tensile strength of 1470 MPa or more, ayield ratio of 0.75 or more and an elongation of 10% or more.

Solution to Problems

In a first invention of the present invention which is anultra-high-strength steel sheet excellent in yield ratio andworkability, the ultra-high-strength steel sheet has a compositioncomprising, by mass %,

C: 0.15% to 0.35%,

Si: 0.5% to 3.0%

Mn: 0.5% to 1.5%,

Al: 0.001% to 0.10% and

the balance being iron and inevitable impurities,

wherein each of P, S and N of the inevitable impurities is limited to

P: 0.1% or less

S: 0.01% or less and

N: 0.01% or less,

the ultra-high-strength steel sheet has a structure comprising, by arearatio based on a whole structure,

martensite: 90% or more and

residual austenite: 0.5% or more,

the ultra-high-strength steel sheet has 1% or more by area ratio of aregion where a local Mn concentration is at least 1.2 times a Mn contentin a whole steel sheet, and

the ultra-high-strength steel sheet has a tensile strength of 1470 MPaor more, a yield ratio of 0.75 or more and an elongation of 10% or more.

In a second invention of the present invention which is theultra-high-strength steel sheet excellent in yield ratio and workabilityaccording to the first invention, the composition further comprises, bymass %, one or two or more of

Cu: 0.05% to 1.0%,

Ni: 0.05% to 1.0% and

B: 0.0002% to 0.0050%.

In a third invention of the present invention which is theultra-high-strength steel sheet excellent in yield ratio and workabilityaccording to the first or second invention, the composition furthercomprises, by mass %, one or two or more of

Mo: 0.01% to 1.0%,

Cr: 0.01% to 1.0%,

Nb: 0.01% to 0.3%,

Ti: 0.01% to 0.3% and

V: 0.01% to 0.3%.

In a fourth invention of the present invention which is theultra-high-strength steel sheet excellent in yield ratio and workabilityaccording to any one of the first to third inventions, the compositionfurther comprises, by mass %, one or two of

Ca: 0.0005% to 0.01% and

Mg: 0.0005% to 0.01%.

Advantageous Effects of Invention

In accordance with the present invention, martensite is used as a mainstructure of steel, and Mn is concentrated in residual austenite,without increasing the average concentration of C and Mn in the wholesteel sheet, whereby it has become possible to provide anultra-high-strength steel sheet that has a high strength and a highyield ratio and is excellent in workability, while ensuring weldability.

DESCRIPTION OF EMBODIMENTS

The present invention will be explained below in greater detail.

First, a structure characterizing an ultra-high-strength steel sheetexcellent in yield ratio and workability in accordance with the presentinvention (hereinafter also referred to as “the steel sheet in thepresent invention”) will be explained.

[Structure of the Steel Sheet in the Present Invention]

As described above, in the steel sheet in the present invention,martensite is used as a matrix, and moreover residual austenite in whichMn is concentrated is contained in a predetermined amount (hereinafter,austenite is sometimes represented by γ).

<Martensite: 90% or More>

In order to realize the steel sheet having a tensile strength of 1470MPa or more and achieve a high yield ratio of 0.75 or more, martensiteis required to be, by area ratio, 90% or more, preferably 92% or more,and more preferably 94% or more. In the present description, martensiteis used to mean including both fresh martensite not subjected totempering and tempered martensite subjected to tempering.

Since all except for residual austenite may be martensite, the upperlimit of the martensite area ratio is 99.5%, and it is preferably 99% orless, in consideration of the lower limit (0.5%) of residual austenite.

<Residual Austenite: 0.5% or More>

In order to use its TRIP action to thereby improve the elongation, theresidual austenite is required to be, by area ratio, 0.5% or more,preferably 0.6% or more, and more preferably 0.7% or more.

Since all except for martensite may be residual austenite, the upperlimit of the residual austenite area ratio is 10%, and it is preferably5% or less, more preferably 3% or less, and particularly preferably 2%or less, in consideration of the lower limit (90%) of martensite.

As described above, although the steel sheet in the present inventionmay be composed of only two phases of martensite and residual austenite(the total area ratio of the two phases is 100%), it is possible toinevitably generate other phases (such as ferrite, bainite andpearlite). The presence of such other phases is allowed as long as thetotal area ratio thereof is 9.5% or less. The total area ratio of theother phases is preferably 7.5% or less, and more preferably 5.5% orless.

<Region Where the Local Mn Concentration is at Least 1.2 Times the MnContent in the Whole Steel Sheet: 1% or More by Area Ratio>

Residual austenite is allowed to remain even in a high strain region byconcentrating Mn in residual austenite to increase stability of theresidual austenite, thereby further improving the elongation to ensurean elongation of 10% or more. On the other hand, from the viewpoint ofensuring weldability, the average Mn concentration in the steel sheet isrequired to fulfill 1.5 mass % or less. In the steel sheet in thepresent invention, therefore, a Mn-concentrated region is formed. Thatis, residual austenite formed in the Mn-concentrated region isstabilized while keeping low the Mn concentration in the matrix. Thisresults in that a part of a region where the local Mn concentration isat least 1.2 times the Mn content in the whole steel sheet is present asresidual austenite to contribute to further improvement of theelongation.

Then, the composition constituting the steel sheet in the presentinvention will be explained. All the units of chemical components arehereinafter by mass %.

[Composition of Steel Sheet in the Present Invention] C: 0.15% to 0.35%

C is an important element having a large influence on the strength ofthe steel sheet. In order to ensure the strength of the steel sheet, Cis contained in an amount of 0.15% or more, preferably 0.16% or more andmore preferably 0.17% or more. However, when C is excessively contained,the weldability is degraded. Therefore, C is contained in an amount of0.35% or less, preferably 0.3% or less, and more preferably 0.25% orless.

Si: 0.5% to 3.0%

Si is a useful element for suppressing the formation of carbides andpromoting the formation of the residual austenite. In order toeffectively exhibit such an action, Si is contained in an amount of 0.5%or more, and is preferably 0.8% or more, and is more preferably 1.1% ormore. However, when Si is excessively contained, the weldability isremarkably degraded. Therefore, Si is contained in an amount of 3.0% orless, preferably 2.5% or less, and more preferably 2.0% or less.

Mn: 0.5% to 1.5%

Mn is a useful element contributing to an increase in the strength ofthe steel sheet as a solid solution hardening element. It has also aneffect of suppressing ferrite transformation during cooling byincreasing hardenability during quenching. In addition, since it hasalso an effect of stabilizing austenite, residual austenite having highstability can be formed. In order to effectively exhibit such actions,Mn is contained in an amount of 0.5% or more, preferably 0.7% or more,and more preferably 0.9% or more. However, the Mn amount is preferablylower from the standpoint of ensuring the weldability, and Mn iscontained in an amount of 1.5% or less, preferably 1.3% or less, andmore preferably 1.15% or less.

Al: 0.001% to 0.10%

Al is a useful element added as a deoxidizing agent, and in order toobtain such an action, it is contained in an amount of 0.001% or more,preferably 0.01% or more, and more preferably 0.03% or more. However,when Al is excessively contained, cleanliness of the steel is degraded.Therefore, Al is contained in an amount of 0.10% or less, preferably0.08% or less, and more preferably 0.06% or less.

The steel sheet in the present invention contains the above-mentionedelements as essential elements, the balance being iron and inevitableimpurities (such as P, S, N and O). Of the inevitable impurities, P, Sand N can be contained up to respective allowable ranges as describedbelow.

P: 0.1% or less

P is inevitably present as an impurity element, and contributes to anincrease in the strength by solid solution hardening. However, thesegregation thereof to prior austenite grain boundary embrittles thegrain boundary, thereby degrading workability. Therefore, the P amountis limited to 0.1% or less, preferably 0.05% or less, and morepreferably 0.03% or less.

S: 0.01% or less

S is also inevitably present as an impurity element, and forms MnSinclusions, which may be starting points of cracks during deformation,thereby decreasing the workability. Therefore, the S amount is limitedto 0.01% or less, preferably 0.005% or less, and more preferably 0.003%or less.

N: 0.01% or less

N is also inevitably present as an impurity element, and decreases theworkability of the steel sheet by strain aging. Therefore, the N amountis limited to 0.01% or less, preferably 0.005% or less, and morepreferably 0.003% or less.

In addition to these, the following allowable components may becontained within the ranges not impairing the actions of the presentinvention.

One or two or more of

Cu: 0.05% to 1.0%, Ni: 0.05% to 1.0% and B: 0.0002% to 0.0050%

These elements are useful elements having an effect of increasinghardenability during quenching and suppressing transformation fromaustenite. In order to obtain such an action, the respective elementsare preferably contained in an amount equal to or more than theabove-mentioned lower limits, respectively. The above-mentioned elementsmay be contained either alone or as a combination of two or morethereof. However, even when these elements are excessively contained,the effect becomes saturated, resulting in an economic waste. Therefore,the respective elements are contained in an amount equal to or less thanthe above-mentioned upper limits, respectively.

One or two or more of

Mo: 0.01% to 1.0%, Cr: 0.01% to 1.0%, Nb: 0.01% to 0.3%, Ti: 0.01% to0.3% and V: 0.01% to 0.3%

These elements are useful for improving the strength without degradingthe workability. In order to obtain such an action, the respectiveelements are preferably contained in an amount equal to or more than theabove-mentioned lower limits, respectively. The above-mentioned elementsmay be contained either alone or as a combination of two or morethereof. However, when these elements are excessively contained, coarsecarbides are formed to degrade the workability. Therefore, therespective elements are contained in an amount equal to or less than theabove-mentioned upper limits, respectively.

One or two of Ca: 0.0005% to 0.01% and Mg: 0.0005% to 0.01%

These elements are useful for improving the workability by decreasingstarting points of fracture by refining inclusions. In order to obtainsuch an action, the elements are each preferably contained in an amountof 0.0005% or more. The above-mentioned elements may be contained eitheralone or as a combination of two of them. However, when excessivelycontained, the inclusions are coarsened on the contrary to degrade theworkability. Therefore, the elements are each contained in an amount of0.01% or less.

Then, preferred production conditions for obtaining the above-mentionedsteel sheet in the present invention will be explained below.

[Preferred Production Method of Steel Sheet in the Present Invention]

First, the steel having the above-mentioned composition is melted, and aslab (steel material) is obtained by ingot making or continuous casting.Thereafter, hot rolling is performed under conditions of a soakingtemperature of 1200° C. or lower (more preferably 1150° C. or lower) anda finishing temperature of 900° C. or lower (more preferably 880° C. orlower), followed by cooling from the finishing temperature to the Ac1point or lower, thereby forming a bainite or pearlite single-phasestructure or a two-phase structure as containing ferrite.

After the above-mentioned hot rolling, annealing treatment is performedunder conditions of holding at 680° C. to the Ac1 point (more preferably690° C. to [Ac1-10° C.]) for 0.8 hours or longer (more preferably 1 houror longer). By this annealing treatment, carbides are spheroidized andcoarsened, and Mn is concentrated in the carbides to at least 1.2 timesthe amount of Mn added to the steel sheet. This annealing treatment maybe performed by holding as such in the above-mentioned temperatureregion after cooling to the Ac1 point or lower, may be performed bygradual cooling in this temperature region, or may be performed afteronce cooled to lower than 680° C. after the hot rolling.

The Ac1 point can be determined from chemical components of the steelsheet using the following formula (1) described in Leslie, “The PhysicalMetallurgy of Steels”, translated by Shigeyasu Kouda, Maruzen, 1985, p.273.

Ac1 (° C.)=723−10.7×Mn−16.9×Ni+29.1×Si+16.9×Cr  (1)

Here, each element symbol in the above-mentioned formula represents thecontent (mass %) of each element.

After the above-mentioned annealed sheet is cold rolled, the cold-rolledsheet is subjected to heat treatment (y-transformation heat treatment)under conditions of holding it at an austenite single-phase regiontemperature (the Ac3 point or higher) for 52 s or longer, therebyaustenitizing the carbides. Since Mn has been concentrated in thecarbides by the annealing treatment in the prior stage, austenite havinga high Mn concentration is formed. By rapid cooling from the austenitesingle-phase region temperature to room temperature at a cooling rate of100° C./s or more, residual austenite where Mn has been concentrated toat least 1.2 times the amount of Mn added to the steel sheet can beformed in martensite that is the matrix.

The Ac3 point can be determined from chemical components of the steelsheet using the following formula (2) described in Leslie, “The PhysicalMetallurgy of Steels”, translated by Shigeyasu Kouda, Maruzen, 1985, p.273.

Ac3 (°C.)=910−203×√C−30×Mn+44.7×Si+700×P+400×Al−15.2×Ni−11×Cr−20×Cu+400×Ti+31.5×Mo+104×V  (2)

Here, each element symbol in the above-mentioned formula represents thecontent (mass %) of each element.

Then, tempered martensite is formed by tempering the above-mentionedheat-treated sheet under conditions of holding it at 150 to 300° C. for30 to 1200 s, and strength-elongation balance can be improved to obtainthe steel sheet in the present invention (the ultra-high-strength steelsheet excellent in the yield ratio and workability).

The present invention will be explained below in greater detail withreference to Examples, but it goes without saying that the presentinvention is not limited to the Examples described below and can beimplemented with appropriate modifications without departing from thespirit described above and later, and all such modification are includedin the technical scope of the present invention.

EXAMPLES [Test Method]

Steels having respective compositions of A to K shown in Table 1described below were melted, and ingots having a thickness of 120 mmwere prepared. Using these ingots, hot rolling was performed to athickness of 2.8 mm, and thereafter, annealing was performed under theannealing conditions shown in Table 2 described below. After theannealed sheets were pickled, they were cold rolled to a thickness of1.0 mm to obtain cold-rolled sheets. Then, the cold-rolled sheets weresubjected to y-transformation heat treatment and tempering under therespective conditions shown in Table 2 described below.

TABLE 1 Transformation Steel Chemical composition* (mass %) temperature(° C.) type C Si Mn Al P S N Others Ac₁ Ac₃ A 0.20 1.78 0.99 0.045 0.0150.0015 0.0041 B: 0.002, Ti: 0.015 743 898 B 0.20 1.84 1.28 0.041 0.0110.0015 0.0042 — 740 887 C 0.19 1.75 1.35 0.045 0.012 0.0012 0.0037 Ca:0.004, Mg: 0.005 739 886 D 0.25 1.20 1.08 0.046 0.008 0.0016 0.0041 Ti:0.05 742 854 E 0.10 1.45 1.02 0.045 0.011 0.0017 0.0038 — 743 906 F 0.221.44 0.49 0.045 0.009 0.0011 0.0035 — 748 889 G 0.21 1.53 0.95 0.0460.013 0.0008 0.0041 Cr: 0.50 752 900 H 0.22 1.64 1.25 0.045 0.010 0.00160.0042 Cu: 0.10 740 874 I 0.21 1.46 1.11 0.045 0.009 0.0012 0.0041 Ni:0.10 740 871 J 0.22 1.39 1.06 0.045 0.016 0.0008 0.0037 Nb: 0.05 742 874K 0.20 1.52 1.08 0.043 0.011 0.0011 0.0041 Mo: 0.10 742 920 L 0.19 1.441.03 0.045 0.009 0.0012 0.0037 V: 0.05 743 884 (Underlined: outside therange of the present invention, *: balance: iron and inevitableimpurities, —: not added)

TABLE 2 Production Steel Annealing after hot rolling γ-transformationheat treatment Tempering No. type Temperature (° C.) Time (h)Temperature (° C.) Time (s) Cooling rate (° C./s) Temperature (° C.)Time (s) 1 A 500 1 930 90 >150 200 360 2 A 700 0.5 930 90 >150 200 360 3A 700 1 930 90 >150 200 360 4 A 700 1 850 90 >150 200 360 5 A 800 1 93090 >150 200 360 6 B 500 1 930 90 >150 200 360 7 B 700 0.5 930 90 >150200 360 8 B 700 1 930 90 >150 200 360 9 B 700 1 850 90 >150 200 360 10 B800 1 930 90 >150 200 360 11 C 700 1 930 90 >150 200 360 12 D 700 1 93090 >150 200 360 13 E 700 1 930 90 >150 200 360 14 F 700 1 930 90 >150200 360 15 G 700 1 930 90 >150 200 360 16 H 700 1 930 90 >150 200 360 17I 700 1 930 90 >150 200 360 18 J 700 1 930 90 >150 200 360 19 K 700 1930 90 >150 200 360 20 L 700 1 930 90 >150 200 360 (Underlined: outsidethe range of the present invention, Hatched: outside the recommendedconditions of the present invention)

[Measurement Methods]

Using each steel sheet obtained, the area ratio of martensite andresidual austenite and the local Mn concentration were measured. Inorder to evaluate mechanical properties of the steel sheet, the yieldstrength (YS), the tensile strength (TS) and the elongation (EL) werealso measured. These measurement methods are shown below.

(Area Ratio of Martensite)

The area ratio of martensite was measured as follows. Each steel sheetwas mirror polished, and a surface thereof was corroded with a 3% Nitalliquid to expose a metal structure. Thereafter, using an SEM (scanningelectron microscope), a structure of a portion of ¼ the sheet thicknesswas observed under a magnification of 2000 for 5 fields of view of anapproximately 40 μm×30 μm region, and a region looking grey was definedas martensite. The area ratios determined for the respective fields ofview were arithmetically averaged as the area ratio of martensite.

(Area Ratio of Residual Austenite)

The area ratio of residual austenite was determined by grinding andpolishing each steel sheet to ¼ the sheet thickness in a sheet thicknessdirection and measuring X-ray diffraction intensity.

(Local Mn Concentration)

The local Mn concentration was determined by quantitatively analyzing 3fields of view of an approximately 20 μm×20 mm region using a fieldemission electron probe microanalyzer (FE-EPMA), dividing a measurementregion to small regions of 1 μm×1 mm in each field of view, andaveraging the Mn concentrations in the respective small regions. Theratio of small regions where the average Mn concentration is at least1.2 times the Mn content in the steel sheet was defined as the arearatio of the Mn-concentrated region in each field of view, andcalculated. Evaluation was performed by arithmetically averaging thearea ratios of the Mn-concentrated regions in the 3 fields of view.

(Yield Strength, Tensile Strength and Elongation)

Using each steel sheet to be evaluated, a No. 5 testpiece described inJIS Z 2201 was prepared while taking a major axis to a directionperpendicular to a rolling direction, and measurement was performed inaccordance with JIS Z 2241 to determine the yield strength (YS), tensilestrength (TS) and elongation (EL), and then, yield ratio (YR) wasdetermined from YS/TS.

[Measurement Results]

The measurement results are shown in Table 3 described below. In theseexamples, the sheet having a tensile strength (TS) of 1470 MPa or more,a yield ratio (YR) of 0.75 or more and an elongation (EL) of 10% or morewas represented by “A” and evaluated as passed, and determined as anultra-high-strength steel sheet that excelled in the yield ratio and theworkability. On the other hand, the sheet having a tensile strength (TS)of less than 1470 MPa, a yield ratio (YR) of less than 0.75 or anelongation (EL) of less than 10% was represented by “B” and determinedas failed.

TABLE 3 Area ratio in structure (%) Mechanical properties Steel SteelProduction Mn-concentrated YS TS YR EL No. type No. Martensite Residualγ region (MPa) (MPa) (−) (%) Evaluation 1 A 1 95 1.1 0.0 1176 1512 0.78 8.2 B 2 A 2 95 1.1 0.6 1174 1513 0.78  9.2 B 3 A 3 95 0.7 1.3 1177 15060.78 10.5 A 4 A 4 84 1.0 1.5 845 1355 0.62 11.4 B 5 A 5 94 0.9 0.0 11681510 0.77  7.9 B 6 B 6 98 1.3 0.0 1194 1535 0.78  7.8 B 7 B 7 97 1.1 0.71187 1533 0.77  8.8 B 8 B 8 98 0.8 1.4 1224 1545 0.79 10.1 A 9 B 9 851.1 1.7 897 1398 0.64 11.2 B 10 B 10 96 1.0 0.0 1187 1520 0.78  7.5 B 11C 11 99 0.8 1.5 1235 1556 0.79 10.2 A 12 D 12 99 0.6 1.1 1156 1487 0.7810.0 A 13 E 13 31 0.0 0.2 412  845 0.49 19.1 B 14 F 14 78 0.0 1.8 7521233 0.61 10.6 B 15 G 15 98 0.7 1.0 1203 1534 0.78 10.5 A 16 H 16 99 0.51.1 1208 1522 0.79 10.3 A 17 I 17 98 0.9 1.3 1194 1535 0.78 10.2 A 18 J18 99 1.0 1.0 1223 1555 0.79 10.1 A 19 K 19 98 1.1 1.4 1234 1565 0.7910.1 A 20 L 20 99 1.0 1.2 1242 1574 0.79 10.3 A (Underlined: outside therange of the present invention, Hatched: outside the recommendedconditions of the present invention)

As shown in Table 3, all the invention steels (steel Nos. 3, 8, 11, 12and 15 to 20) fulfilling the requirements of the present invention (theabove-mentioned component requirements and the above-mentioned structurerequirements) satisfy a tensile strength TS of 1470 MPa or more, a yieldratio YR of 0.75 or more and an elongation EL of 10% or more, and theultra-high-strength steel sheets excellent in the yield ratio and theworkability have been obtained.

By contrast, the comparative steels (steel Nos. 1, 2, 4 to 7, 9, 10, 13and 14) not satisfying at least one of the requirements of the presentinvention (the above-mentioned component requirements and theabove-mentioned structure requirements) are degraded in at least any oneproperty of the tensile strength TS, the yield ratio YR and theelongation EL.

For example, in steel Nos. 1 and 6, the annealing temperature after hotrolling is too low and is outside the recommended range as shown inproduction Nos. 1 and 6 of Table 2, respectively. Thus, Mn is notsufficiently concentrated in residual austenite to degrade theelongation EL, as shown in Table 3.

On the other hand, in steel Nos. 5 and 10, the annealing temperatureafter hot rolling is too high and is outside the recommended range asshown in production Nos. 5 and 10 of Table 2, respectively. Thus, Mn ishomogenized by diffusion, and Mn is not sufficiently concentrated inresidual austenite to degrade the elongation EL, as shown in Table 3.

Further, in steel Nos. 2 and 7, the annealing holding time after hotrolling is too short and is outside the recommended range as shown inproduction Nos. 2 and 7 of Table 2, respectively. Thus, Mn is notsufficiently concentrated in residual austenite to degrade theelongation EL, as shown in Table 3.

In addition, in steel Nos. 4 and 9, the y-transformation heat treatmenttemperature is too low and is outside the recommended range as shown inproduction Nos. 4 and 9 of Table 2, respectively. Thus, austenitizationis not sufficiently achieved, and martensite is insufficient, resultingin poor tensile strength TS and yield ratio YR, as shown in Table 3.

Furthermore, in steel No. 13, the C content is too low as shown in steeltype E of Table 1. Thus, both martensite and residual austenite areinsufficient, and Mn is not sufficiently concentrated in residualaustenite, resulting in poor tensile strength TS and yield ratio YR, asshown in Table 3.

Further, in steel No. 14, the Mn content is too low as shown in steeltype F of Table 1. Thus, both martensite and residual austenite areinsufficient, resulting in poor tensile strength TS and yield ratio YR,as shown in Table 3.

As described above, it has been confirmed that the ultra-high-strengthsteel sheets excellent in the yield ratio and the workability areobtained by satisfying the requirements of the present invention.

While the present invention has been described in detail and withreference to specific embodiments thereof, it will be apparent to oneskilled in the art that various changes and modifications can be madetherein without departing from the spirit and scope thereof.

This application is based on Japanese Patent Application No. 2015-026736filed on Feb. 13, 2015, the entire subject matter of which isincorporated herein by reference.

INDUSTRIAL APPLICABILITY

The ultra-high-strength steel sheet of the present invention isexcellent in yield ratio and workability, and is useful for vehiclebodies as cold-rolled steel sheets and various plated steel sheets.

1. An ultra-high-strength steel sheet excellent in yield ratio andworkability, having a composition comprising, by mass %, C: 0.15% to0.35%, Si: 0.5% to 3.0% Mn: 0.5% to 1.5%, Al: 0.001% to 0.10% and thebalance being iron and inevitable impurities, wherein each of P, S and Nof the inevitable impurities is limited to P: 0.1% or less S: 0.01% orless and N: 0.01% or less, the ultra-high-strength steel sheet having astructure comprising, by area ratio based on a whole structure,martensite: 90% or more and residual austenite: 0.5% or more, theultra-high-strength steel sheet having 1% or more by area ratio of aregion where a local Mn concentration is at least 1.2 times a Mn contentin a whole steel sheet, and the ultra-high-strength steel sheet having atensile strength of 1470 MPa or more, a yield ratio of 0.75 or more andan elongation of 10% or more.
 2. The ultra-high-strength steel sheetexcellent in yield ratio and workability according to claim 1, whereinthe composition further comprises, by mass %, at least one of thefollowing (a) to (c): (a) one or two or more of Cu: 0.05% to 1.0%, Ni:0.05% to 1.0% and B: 0.0002% to 0.0050%, (b) one or two or more of Mo:0.01% to 1.0%, Cr: 0.01% to 1.0%, Nb: 0.01% to 0.3%, Ti: 0.01% to 0.3%and V: 0.01% to 0.3%, and (c) one or two of Ca: 0.0005% to 0.01% and Mg:0.0005% to 0.01%.